1. Field of the Invention
This invention relates to a semiconductor device, and more particularly, to a process for hermetically sealing a semiconductor device comprising a package base, on which a semiconductor chip is mounted, and a package cap which is secured and sealed to the base by a resin adhesive.
2. Description of the Related Art
A process for hermetically sealing a package cap onto a package base, on which a semiconductor chip is mounted, by a resin adhesive is well known in the prior art, and such a known process comprises the following steps:
First, a package base is provided with a cavity on which a semiconductor chip is mounted by a resin adhesive in a die-bonding process. The semiconductor chip has a plurality of electric terminals which are connected to inner leads, respectively, of the package base by bonding wires in a wire-bonding process.
The package base is provided at the outer periphery of the cavity with a flat portion which is coated with a thermosetting resin adhesive, and a package cap is then placed on the package base and positioned thereon. The cap is pressed against the package base by a suitable means, such as a pressure clip, to press the cap and the base together in such a way that they can not move relative to each other.
Then, the package comprising the base and cap is introduced into a furnace and the package is heated at a constant temperature, for example, approximately 150.degree. C., to thermoset the resin adhesive, and after a predetermined time, the cap becomes firmly adhered to the base and the package is removed from the furnace. The pressure clip is then removed from the package, and an hermetic sealed semiconductor device is obtained.
The above-mentioned known process for hermetically sealing a package cap onto a package base, however, has several disadvantages, as mentioned below.
The pressure clip is set on the package at a room temperature, such as 20.degree. C. and under atmospheric pressure, and thus the cap is sealed to the package base by an adhesive resin at an ambient pressure. Therefore, after the package is placed in the furnace in which a constant temperature, for example, approximately 150.degree. C., is maintained, the pressure in the cavity of the package becomes higher than the ambient pressure due to an expansion of the air confined in the cavity. For example, assuming that the room temperature is 20.degree. C. and the temperature of the furnace is 150.degree. C., the pressure in the cavity becomes 1.44 times the ambient pressure.
A pressure difference between a high pressure in the package cavity and the ambient pressure in the furnace causes the resin adhesion to be urged toward the outside of the package, and accordingly, the resin adhesive contact area between the package base and the cap is reduced or pin holes are created in the resin adhesive which causes various problems. For example, the sealing force of the resin adhesive is reduced.